Water-based coating composition

ABSTRACT

The present invention provides an aqueous coating composition comprising a carboxyl-containing, acrylic-modified polyester resin (B) neutralized and dispersed in an aqueous medium, the polyester resin (B) being obtainable by reacting a carboxyl-containing acrylic resin (c) with an epoxy-modified polyester resin (A) prepared by esterification of a carboxyl-containing polyester resin (a) having a number average molecular weight of 1,000 to 30,000 and an acid value of 5 to 200 mg KOH/g with a 1,2-epoxy-containing low-molecular-weight epoxy compound (b) having a number average molecular weight of 1,200 or less.

TECHNICAL FIELD

The present invention relates to an aqueous coating composition, inparticular a novel aqueous coating composition free of the problems ofenvironmental hormones and suitable for coating the inner surfaces ofcans.

BACKGROUND OF THE INVENTION

In order to solve the problem of global environmental pollution causedby the evaporation of organic solvents, aqueous coating compositionshave been replacing organic solvent-based coating compositions in thefield of coating compositions for the inner surfaces of cans such asfood cans. Japanese Examined Patent Publication No. 1988-41934 andJapanese Unexamined Patent Publication No. 1995-138523 propose, as theaqueous coating compositions, compositions comprising an epoxy-modifiedacrylic resin prepared by reacting or mixing an epoxy resin with acarboxyl-containing acrylic resin.

In these known aqueous coating compositions, a high-molecular-weightepoxy resin obtained by reacting a low-molecular-weight epoxy resin withbisphenol A is generally used as the starting epoxy resin, in order toachieve good application workability and high film performance. As aresult, unreacted bisphenol A remains in the compositions and causes theproblem of environmental hormones. However, if a low-molecular-weightepoxy resin in which no bisphenols are detected is used as the startingepoxy resin to avoid the problem, the resulting aqueous coatingcomposition is poor in application workability and film performancecharacteristics such as adhesion to substrates, flavor protectingproperties, etc., and thus is unsuitable as a coating composition forthe inner surfaces of cans.

Therefore, no fundamental solution has been proposed so far to avoid theproblem of environmental hormones caused by bisphenols in aqueouscoating compositions comprising an epoxy-modified acrylic resin.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a novel aqueous coatingcomposition free of the problems of the prior art.

Another object of the invention is to provide a novel aqueous coatingcomposition which is free of the problem of environmental hormonescaused by bisphenols while having good application workability and highfilm performance.

Other objects and features of the invention will be apparent from thefollowing description.

The present invention provides an aqueous coating composition comprisinga carboxyl-containing, acrylic-modified polyester resin (B) neutralizedand dispersed in an aqueous medium, the polyester resin (B) beingobtainable by reacting a carboxyl-containing acrylic resin (c) and anepoxy-modified polyester resin (A) prepared by esterification of acarboxyl-containing polyester resin (a) having a number averagemolecular weight of 1,000 to 30,000 and an acid value of 5 to 200 mgKOH/g with a 1,2-epoxy-containing low-molecular-weight epoxy compound(b) having a number average molecular weight of 1,200 or less.

The inventors carried out extensive research to develop an aqueouscoating composition which is free of the problem of environmentalhormones caused by bisphenols and which is suitable for coating theinner surfaces of cans. As a result, the inventors found that when anaqueous coating composition is prepared by neutralizing and dispersing aresin obtained by sequentially reacting a specific carboxyl-containingpolyester resin, a specific low-molecular-weight epoxy compound in whichno bisphenols are detected, and a carboxyl-containing acrylic resin asstarting materials, the aqueous coating composition contains no freebisphenols and is capable of achieving the same application workabilityand film performance as attained by known aqueous coating compositionscomprising an epoxy-modified acrylic resin. The present invention hasbeen accomplished based on these new findings.

The epoxy-modified polyester resin (A) in the aqueous coatingcomposition of the invention is prepared by esterification of acarboxyl-containing polyester resin (a) having a number averagemolecular weight of 1,000 to 30,000 and an acid value of 5 to 200 mgKOH/g, and a 1,2-epoxy-containing low-molecular-weight epoxy compound(b) having a number average molecular weight of 1,200 or less.

It is essential that the carboxyl-containing polyester resin (a) have anumber average molecular weight of 1,000 to 30,000, and an acid value of5 to 200 mg KOH/g. If the number average molecular weight is less than1,000 or the acid value exceeds 200 mg KOH/g, side reactions are likelyto occur at the time of addition reaction of the carboxyl group and theepoxy group of the epoxy compound (b), increasing the viscosity of theresulting resin (A) and thereby making the production of the compositiondifficult. On the other hand, a number average molecular weightexceeding 30,000 or an acid value less than 5 mg KOH/g makes itdifficult to disperse the polyester resin (B) in an aqueous medium inthe following step. Thus, molecular weights and acid values outside theabove-specified range are undesirable.

Preferably, the carboxyl-containing polyester resin (a) has a numberaverage molecular weight of 2,000 to 10,000, and an acid value of about20 to 150 mg KOH/g.

The polyester resin (a) is prepared by polycondensation of a polyhydricalcohol with a polybasic acid. Polyhydric alcohols are polyols such asalkane polyols, oxyalkylene polyols, polyoxyalkylene polyols, alicyclicpolyols and the like. Typical examples of polyhydric alcohols includeethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol,1,4-butanediol, 1,3-butanediol, 2-methyl-1,3-propanediol,1,5-pentanediol, 1,3-pentanediol, 1,6-hexanediol, 1,5-hexanediol,neopentyl glycol, glycerin, 2-methylglycerin, trimethylolpropane,trimethylolethane, pentaerythritol and like alkane polyols; diethyleneglycol, dipropylene glycol and like oxyalkylene polyols; triethyleneglycol, tetraethylene glycol, polyethylene glycol, tripropylene glycol,polypropylene glycol and like polyoxyalkylene polyols; 1,4-cyclohexanedimethanol and like alicyclic polyols; etc. Examples of polybasic acidsinclude adipic acid, sebacic acid, glutaric acid, phthalic acid,isophthalic acid, terephthalic acid, maleic acid, fumaric acid, succinicacid, pimelic acid, azelaic acid, dodecanedioic acid,cyclohexanedicarboxylic acid, tetrahydrophthalic acid,methyltetrahydrophthalic acid, endomethylene-tetrahydrophthalic acid,methylendomethylene-tetrahydrophthalic acid, hexahydrophthalic acid,naphthalenedicarboxylic acid, trimellitic acid, butanetricarboxylicacid, anhydrides of these acids, and the like.

In the polycondensastion of a polyhydric alcohol with a polybasic acid,a strong protonic acid, a heavy metal oxide or the like can be used as apolycondensation catalyst. Examples of strong protonic acids includesulfuric acid, benzenesulfonic acid, p-toluenesulfonic acid and thelike. Examples of heavy metal oxides include tetrabutyl titanate,dibutyltin oxide, antimony trioxide, manganese dioxide and the like.

The polyester resin (a) can be prepared by subjecting a polyhydricalcohol and a polybasic acid to condensation so as to introduce carboxylgroups into the resin molecule and thereby impart the specific acidvalue required for the invention. Alternatively, the resin (a) may beobtained by first preparing a hydroxyl-containing polyester resin,followed by addition of an acid anhydride to the hydroxyl groups toimpart the specific acid value. Examples of the acid anhydrides includephthalic anhydride, trimellitic anhydride, succinic anhydride, maleicanhydride, hexahydrophthalic anhydride, pyromellitic anhydride and thelike.

It is essential that the 1,2-epoxy-containing low-molecular-weight epoxycompound (b) have a number average molecular weight of 1,200 or less.The low-molecular-weight epoxy compound, in which no unreactedbisphenols are detected, is used to modify the carboxyl-containingpolyester resin (a). As a result, an aqueous coating composition can beobtained which contains no free bisphenols while having good applicationworkability and high film performance. Therefore, the obtained aqueouscoating composition is free of the problem of environmental hormones.

The epoxy compound (b) preferably has an epoxy equivalent of 140 to 250,and a number average molecular weight of 250 to 900.

Compounds usable as the epoxy compound (b) include, for example,bisphenol A diglycidyl ether and like bisphenol A epoxy resins;bisphenol F diglycidyl ether and like bisphenol F epoxy resins; dimeracid diglycidyl ester, polypropylene glycol diglycidyl ether,hydrogenated bisphenol A diglycidyl ether, epoxycyclohexylmethylepoxycyclohexanecarboxylate, epoxy phenol novolac, triglycidylisocyanurate like non-bisphenol epoxy resins; etc.

The epoxy-modified polyester resin (A) in the composition of theinvention can be preferably prepared by esterification of thecarboxyl-containing polyester resin (a) with the low-molecular-weightepoxy compound (b) in such a ratio that the molar ratio of the carboxylgroups in the resin (a) to the epoxy groups in the compound (b) is 1:2to 1:1. The epoxy-modified polyester resin (A) thus obtained has anepoxy equivalent of about 2,000 to 10,000, and a number averagemolecular weight of about 4,000 to 100,000. It is suitable that theepoxy-modified polyester resin (A) have an epoxy equivalent and numberaverage molecular weight within the above ranges, from the viewpoints ofthe stability, application workability and film performance of theresulting aqueous coating composition.

Preferably, the polyester resin (A) has an epoxy equivalent of about2,500 to 7,000, and a number average molecular weight of about 5,000 to50,000.

The esterification of the polyester resin (a) with the epoxy compound(b) can be effected by, for example, heating the mixture of the resin(a) and compound (b) at 120 to 150° C for about 1 to 8 hours in thepresence of an organic solvent and a catalyst such as tertiaryalkylamine, tetraalkylammonium halide or the like, to thereby obtain thedesired resin (A).

Usually, the polyester resin (A) is obtained by addition reaction of theterminal carboxyl group of the polyester resin (a) and the terminal1,2-epoxy group of the epoxy compound (b), and thus has a hydroxyl groupin the side chain of the molecule. Since the hydroxyl group in the sidechain acts to increase the adhesion to metal substrates, the presence ofthe hydroxyl group is advantageous in improving the adhesion tosubstrates and corrosion resistance of the resulting coating film.

The carboxyl-containing acrylic resin (c) to be reacted with thepolyester resin (A) for preparing the carboxyl-containing,acrylic-modified polyester resin (B) is an acrylic polymer comprising,as an essential monomer component, acrylic acid, methacrylic acid,itaconic acid, fumaric acid or like polymerizable unsaturated carboxylicacid. It is preferable that the polymer have an acid value of 100 to 500mg KOH/g, from the viewpoints of the stability of the resin (B) in theaqueous medium, and the processability, corrosion resistance and waterresistance of the resulting coating film, etc.

Examples of monomer components other than the polymerizable unsaturatedcarboxylic acid for preparation of the acrylic resin (c) include methyl(meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl(meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate,2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, stearyl(meth)acrylate, cetyl (meth)acrylate, and like C₁₋₁₈ alkyl esters ofacrylic or methacrylic acid; benzyl (meth)acrylate, cyclohexyl(meth)acrylate, isobornyl (meth)acrylate; styrene, a-methylstyrene,vinyltoluene and like aromatic vinyl monomers; hydroxyethyl(meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl(meth)acrylate, hydroxyamyl (meth)acrylate, hydroxyhexyl (meth)acrylate,and like hydroxyalkyl (meth)acrylates; hydroxyl-containing,caprolactone-modified alkyl (meth)acrylate prepared by ring-openingaddition reaction of 1 to 5 moles of ε-caprolactone to 1 mol of one ofthe above hydroxyalkyl (meth)acrylates, and like hydroxyl-containingpolymerizable unsaturated monomers; acrylamide, methacrylamide,N-methoxymethyl (meth)acrylamide, N-ethoxymethyl (meth)acrylamide,N-n-propoxymethyl (meth)acrylamide, N-isopropoxymethyl (meth)acrylamide,N-n-butoxymethyl (meth)acrylamide, N-sec-butoxymethyl (meth)acrylamide,N-tert-butoxymethyl (meth)acrylamide and like acrylamide monomers;acrylonitrile, methacrylonitrile, vinyl acetate, ethylene and butadiene;and the like.

The acrylic resin (c) can be obtained by, for example, heating a monomermixture of a polymerizable unsaturated carboxylic acid and other monomercomponent(s) at 80 to 150° C. for about 1 to 10 hours in an organicsolvent in the presence of a radical polymerization initiator or chaintransfer agent, to thereby effect copolymerization. As thepolymerization initiator, an organic peroxide polymerization initiatoror azo polymerization initiator can be used. Examples of organicperoxide polymerization initiators include benzoyl peroxide,t-butylperoxy-2-ethylhexanoate, di-t-butylperoxide,t-butylperoxybenzoate and t-amylperoxy-2-ethylhexanoate. Examples of azopolymerization initiators include azobisisobutyronitrile andazobisdimethyl-valeronitrile. The chain transfer agent may be, forexample, α-methylstyrene dimer, a mercaptan or the like.

The reaction of the epoxy-modified polyester resin (A) and thecarboxyl-containing acrylic resin (c) can be carried out usually byheating a mixture of the resins at about 80 to 120° C. for about 0.5 to8 hours in an organic solvent in the presence of a catalyst such astriethylamine, dimethylethanolamine or like tertiary amine ortriphenylphosphine or like quaternary salt compound. Thus, thecarboxyl-containing, acrylic-modified polyester resin (B) can bepreferably prepared. When the polyester resin (A) and the acrylic resin(c) are reacted with each other, conversion to an onium salt or likereaction usually occurs together with the esterification as the mainreaction.

The proportions of the polyester resin (A) and the carboxyl-containingacrylic resin (c) for use in the above reaction can be selected so as toachieve good application workability, high film performance, etc. It isusually preferable that the weight ratio of the resin (A) to the resin(c) be 60/40 to 90/10, more preferably 70/30 to 90/10, based on solids.

The carboxyl-containing, acrylic-modified polyester resin (B) obtainedby the above reaction has an acid value of preferably 15 to 200 mgKOH/g, from the viewpoints of the dispersion stability in an aqueousmedium, the water resistance of the resulting coating film, etc.Further, it is usually desirable that the resin (B) have substantiallyno epoxy group, from the viewpoint of storage stability of the resultingcomposition.

The polyester resin (B) is neutralized and dispersed in an aqueousmedium. For the neutralization, a neutralizing agent such as an amine,ammonia or the like is preferably usable. Typical examples of aminesinclude triethylamine, triethanolamine, dimethylethanolamine,diethylethanolamine, morpholine and the like. Among these amines,triethylamine and dimethylethanolamine are particularly preferable. Thedegree of neutralization of the polyester resin (B) is not limited, butneutralization with 0.3 to 1.0 equivalent relative to the carboxylgroups in the resin (B) is preferable.

The aqueous medium in which the polyester resin (B) is to be dispersedmay be water or a mixture of water and an organic solvent. The organicsolvent may be any of known organic solvents, as long as it does notadversely affect the stability of the polyester resin (B) in the aqueousmedium and is miscible with water.

Preferably, the organic solvent is an alcohol solvent, cellosolvesolvent, carbitol solvent or the like. Specific examples of the organicsolvent include n-butanol and like alcohol solvents; ethylene glycolmonobutyl ether, ethylene glycol monoisopropyl ether, ethylene glycolmonomethyl ether, propylene glycol monomethyl ether and like cellosolvesolvents; and diethylene glycol monoethyl ether and like carbitolsolvents. Further, an inert organic solvent immiscible with water may beused within a range that does not adversely affect the stability of theacrylic-modified polyester resin in an aqueous medium. Examples of suchorganic solvents include toluene, xylene and like aromatic hydrocarbonsolvents; ethyl acetate, butyl acetate and like ester solvents; andmethyl ethyl ketone and like ketone solvents. In the composition of theinvention, the proportion of the organic solvent in the aqueous mediumis preferably not greater than 50 wt. %, in consideration of theenvironmental protection.

The polyester resin (B) can be neutralized and dispersed in an aqueousmedium by a know method. For example, the polyester resin (B) may begradually added, with stirring, to an aqueous medium containing aneutralizing agent; or the polyester resin (B) may be first neutralizedwith a neutralizing agent, followed by addition of an aqueous medium tothe neutralized resin with stirring, or addition of the neutralizedresin to an aqueous medium with stirring.

The composition of the invention may further comprise a crosslinkingagent, in addition to the neutralized polyester resin (B). Usablecrosslinking agents include resol type phenol resins, melamine resins,benzoguanamine resins and the like. Examples of phenol components in thephenol resins include o-cresol, p-cresol, p-tert-butylphenol,p-ethylphenol, 2,3-xylenol, 2,5-xylenol, p-tert-amylphenol,p-nonylphenol, p-cyclohexylphenol and like bifunctional phenols;carbolic acid, m-cresol, m-ethylphenol, 3,5-xylenol, m-methoxphenol andlike trifunctional phenols; 2,4-xylenol, 2,6-xylenol and likemonofunctional phenols; bisphenol B, bisphenol F and liketetrafunctional phenols; etc. These components may be used either singlyor in combination. It is preferable to avoid the use of bisphenolresins, since they are likely to cause the problem of environmentalhormones. Further, the composition of the invention may contain asurfactant, a defoaming agent or the like, if necessary.

The aqueous coating composition of the invention preferably has a solidcontent of about 15 to 40 wt. %.

The composition of the invention is suitable for coating the innersurfaces of cans such as food cans. The composition is applicable tovarious metal substrates, such as aluminum plates, steel plates and likemetal plates; steel plates coated with zinc, chromium, tin, aluminium orthe like; and steel plates whose surfaces have been subjected tochemical conversion with chromic acid, iron phosphate, zinc phosphate orthe like. The composition can be applied to the surface of a metalsubstrate by a known method such as roll coating, spray coating, brushcoating, electrodeposition coating or the like. A coating thickness of 2to 30 μm is usually sufficient. The coating is baked usually at about150 to 280° C., preferably about 180 to 220° C., for about 20 to 600seconds, preferably about 30 to 300 seconds.

BEST MODE FOR CARRYING OUT THE INVENTION

The following Production Examples, Examples and Comparative Examples areprovided to illustrate the invention in further detail. In theseexamples, parts and percentages are all by weight.

PRODUCTION EXAMPLE 1

Production of Carboxyl-containing Acrylic Resin

Ethylene glycol monobutyl ether (1,200 parts) was placed in a reactionvessel, heated to 100° C. and maintained at the temperature. A mixtureof methacrylic acid (400 parts), styrene (500 parts), ethyl acrylate(100 parts), “Perbutyl O ” (a product of NOF CORPORATION, a peroxidepolymerization initiator) (35 parts) and ethylene glycol monobutyl ether(140 part) was added dropwise over 3 hours. After completion of theaddition, the resulting mixture was aged at 100° C. for 2 hours. Then,570 parts of n-butanol was added, giving a carboxyl-containing acrylicresin solution having a solid content of 36%. The resin had a numberaverage molecular weight of about 7,000 and an acid value of 260 mgKOH/g.

EXAMPLE 1

Ethylene glycol (310 parts), propylene glycol (380 parts), adipic acid(234 parts), isophthalic acid (580 parts), terephthalic acid (664parts), cyclohexanone (100 parts) and dibutyltin oxide (0.5 parts) wereplaced in a reaction vessel, heated to 230° C. over 3 hours withstirring, and maintained at the temperature for 5 hours. After coolingthe mixture to 160° C., 25 parts of phthalic anhydride was added, andthe resulting mixture was maintained at 160° C. for 2 hours, giving apolyester resin solution. The resin had an acid value of 50 mg KOH/g anda number average molecular weight of about 4,000.

Into 1,000 parts of the polyester resin solution were added bisphenol Adiglycidyl ether (epoxy equivalent: about 190; number average molecularweight: about 350; no residual bisphenol A was detected therein) (240parts), cyclohexanone (210 parts) and tri-n-butylamine (0.5 parts). Theresulting mixture was maintained at 130° C. for 5 hours, giving anepoxy-modified polyester resin solution. The resin had an epoxyequivalent of about 3,000 and a number average molecular weight of about20,000.

Subsequently, the carboxyl-containing acrylic resin solution with asolid content of 36% obtained in Production Example 1 (870 parts) andethylene glycol monobutyl ether (220 parts) were added to theepoxy-modified polyester resin solution, followed by uniform stirring.The mixture was cooled to 85° C., and after adding 77 parts ofdimethylethanolamine, the resulting mixture was maintained at 85° C. for1 hour. Then, 3,600 parts of deionized water was added dropwise over 1hour, to obtain an aqueous coating composition of the invention as anaqueous dispersion of a resin with an acid value of 37 mg KOH/g and aparticle size of 200 nm, the aqueous dispersion having a solid contentof 25% and a viscosity of 4,000 mPas.

EXAMPLE 2

Ethylene glycol (124 parts), propylene glycol (152 parts), adipic acid(182 parts), isophthalic acid (500 parts), terephthalic acid (660parts), cyclohexanone (100 parts) and dibutyltin oxide (0.5 parts) wereplaced in a reaction vessel, heated to 230° C. over 3 hours withstirring, and maintained at the temperature for 8 hours, to obtain apolyester resin solution. The resin had an acid value of 40 mg KOH/g anda number average molecular weight of about 5,000.

Into 1,000 parts of the polyester resin solution were added bisphenol Adiglycidyl ether (epoxy equivalent: about 190; number average molecularweight: about 350; no residual bisphenol A was detected therein) (200parts), cyclohexanone (200 parts) and tri-n-butylamine (0.5 parts). Themixture was maintained at 130° C. for 3 hours with stirring, giving anepoxy-modified polyester resin solution. The resin had an epoxyequivalent of about 3,000 and a number average molecular weight of about20,000.

Then, the carboxyl-containing acrylic resin solution with a solidcontent of 36% obtained in Production Example 1 (840 parts) and ethyleneglycol monobutyl ether (220 parts) were added to the epoxy-modifiedpolyester resin solution, followed by uniform stirring. The mixture wascooled to 85° C., and after adding 75 parts of dimethylethanolamine, theresulting mixture was maintained at 85° C. for 1 hour. Thereafter, 3,500parts of deionized water was added dropwise over 1 hour, to obtain anaqueous coating composition of the invention as an aqueous dispersion ofa resin with an acid value of 37 mg KOH/g and a particle size of 250 nm,the aqueous dispersion having a solid content of 25% and a viscosity of3,000 mPas.

EXAMPLE 3

Ethylene glycol (155 parts), propylene glycol (190 parts), adipic acid(230 parts), isophthalic acid (530 parts), terephthalic acid (750parts), cyclohexanone (100 parts) and dibutyltin oxide (0.5 parts) wereplaced in a reaction vessel, heated to 230° C. over 3 hours withstirring, and maintained at the temperature for 8 hours. The mixture wascooled to 160° C., and after adding 8 parts of phthalic anhydride, theresulting mixture was maintained at 160° C. for 2 hours, giving apolyester resin solution. The resin had an acid value of 20 mg KOH/g anda number average molecular weight of about 6,000.

Into 1,000 parts of the polyester resin solution were added bisphenol Adiglycidyl ether (epoxy equivalent: about 190; number average molecularweight: about 350; no residual bisphenol A was detected therein) (120parts), cyclohexanone (200 parts) and tri-n-butylamine (0.5 parts). Themixture was maintained at 130° C. for 3 hours, giving an epoxy-modifiedpolyester resin solution. The resin had an epoxy equivalent of about3,000 and a number average molecular weight of about 20,000.

Subsequently, the carboxyl-containing acrylic resin solution with asolid content of 36% obtained in Production Example 1 (780 parts) andethylene glycol monobutyl ether (220 parts) were added to theepoxy-modified polyester resin solution, followed by uniform stirring.The mixture was cooled to 85° C., and after adding 70 parts ofdimethylethanolamine, the resulting mixture was maintained at 85° C. for1 hour. Subsequently, 3,200 parts of deionized water was added dropwiseover 1 hour, to thereby obtain an aqueous coating composition of theinvention as an aqueous dispersion of a resin with an acid value of 37mg KOH/g and a particle size of 300 nm, the aqueous dispersion having asolid content of 25% and a viscosity of 3,000 mPas.

COMPARATIVE EXAMPLE 1

Into a reaction vessel were placed bisphenol A diglycidyl ether (epoxyequivalent: about 190; number average molecular weight: about 350; noresidual bisphenol A was detected therein) (1,000 parts), bisphenol A(556 parts), ethylene glycol monobutyl ether (172 parts) and 50% aqueoustetramethylammonium solution (1.6 parts). The mixture was heated to 140°C. with stirring and maintained at the temperature for 5 hours, giving ahigh-molecular-weight epoxy resin solution. The resin had a solidcontent of 90%, an epoxy equivalent of 3,000 and a number averagemolecular weight of 8,000.

Subsequently, the carboxyl-containing acrylic resin solution having asolid content of 36% obtained in Production Example 1 (630 parts) andethylene glycol monobutyl ether (110 parts) were added to 1,000 parts ofthe epoxy resin solution, followed by uniform stirring. The mixture wascooled to 85° C., and after adding 50 parts of deionized water and 60parts of dimethylethanolamine, the resulting mixture was maintained at85° C. for 1 hour. Thereafter, 2,600 parts of deionized water was addeddropwise over 1 hour, to obtain a comparative aqueous coatingcomposition as an aqueous dispersion of a resin with an acid value of 36mg KOH/g and a particle size of 180 nm, the aqueous dispersion having asolid content of 25% and a viscosity of 3,000 mPas.

COMPARATIVE EXAMPLE 2

Into a reaction vessel were placed bisphenol A diglycidyl ether (epoxyequivalent: about 190; number average molecular weight: about 350; noresidual bisphenol A was detected therein) (1,800 parts), bisphenol A(1,487 parts), diethylene glycol monobutyl ether (363 parts) and a 50%aqueous tetramethylammonium solution (1 part). The mixture was heated to140° C. with stirring, and maintained at the temperature for 8 hours,giving a high-molecular-weight epoxy resin solution. The resin had asolid content of 90%, an epoxy equivalent of 9,100, and a number averagemolecular weight of 5,500.

Then, the carboxyl-containing acrylic resin solution with a solidcontent of 36% obtained in Production Example 1 (630 parts) and ethyleneglycol monobutyl ether (113 parts) were added to 1,000 parts of theepoxy resin solution, followed by uniform stirring. The mixture wascooled to 85° C., and after adding deionized water (18 parts) anddimethylethanolamine (60 parts), the resulting mixture was maintained at85° C. for 1 hour. Thereafter, 2,600 parts of deionized water was addeddropwise over 1 hour, to obtain a comparative aqueous coatingcomposition as an aqueous dispersion of a resin with an acid value of 36mg KOH/g and a particle size of 350 nm, the aqueous dispersion having asolid content of 25% and a viscosity of 800 mPas. Performance test ofaqueous coating composition

The aqueous coating compositions obtained in Examples 1 to 3 andComparative Examples 1 and 2 were applied by spray coating to clean,tin-free steel plates with a thickness of 0.23 mm, to a dry thickness of10 μm, and baked at 200° C. for 3 minutes for curing. Using the coatedplates, film performance characteristics were tested by the followingmethods. Also, the bisphenol A concentration in each aqueous coatingcomposition was measured.

Test Methods

Gel fraction: Each of the cured coating films was subjected toextraction with refluxing acetone for 6 hours to find the percentage byweight of the non-extracted portion of the coating film afterextraction, relative to the coating film before extraction.

Adhesion to substrate: A nylon film was sandwiched between two coatedplates (150 mm×5 mm) in such a manner that the nylon film came intocontact with the coated surfaces. The coated plates with the nylon filmwas heated at 200° C. for 60 seconds, compressed at 200° C. for 30seconds to fuse the nylon film to the coating films, and used as a testpiece. The T-peel adhesion strength of the test piece was measured usinga tensile tester (tradename: “Autograph AGS-500A”, manufactured byShimadzu Corporation). The tensile test was carried out at 20° C. with apulling speed of 200 mm/min.

Processability: The coated plates were cut to a size of 40 mm×50 mm, andbent in half in such a manner that the coated surfaces faced outwards. A3 kg weight was dropped onto the bent part of each plate from a heightof 42 cm. A 20 mm long portion from the bent part was dipped in a 1%aqueous sodium chloride solution, and a voltage (6.5 V) was applied tomeasure the current. The smaller the current, the better theprocessability.

Corrosion resistance: The coated plates were cut to a size of 150 mm×70mm, and crosswise cuts reaching the substrates were made on the coatedplates. The coated plates were subjected to a 3-week salt spray test,and then visually inspected and rated on the following scale.

A: The width of the rusted portion on each side of the cuts was lessthan 2 mm.

B: The width of the rusted portion on each side of the cuts was not lessthan 2 mm but less than 5 mm.

C: The width of the rusted portion on each side of the cuts was 5 mm ormore.

Acid resistance: The coated plates whose back faces and side faces hadbeen sealed were dipped in a 10% aqueous hydrochloric acid solution at20° C. for 1 week. Then, the coated surfaces were visually inspected andrated on the following scale.

A: No abnormalities.

B: Slight blushing.

C: Serious blushing.

Alkali resistance: The coated plates whose back faces and side faces hadbeen sealed were dipped in a 10% aqueous caustic soda solution at 20° C.for 1 week. Then, the coated surfaces were visually inspected and ratedon the following scale.

A: No abnormalities.

B: Slight blushing.

C: Serious blushing.

Bisphenol A concentration: The aqueous coating compositions obtained inExamples and Comparative Examples were dissolved in tetrahydrofuran, andanalyzed by high performance liquid chromatography to determine thebisphenol A concentration in each composition.

Table 1 shows the results.

TABLE 1 Comparative Example Example 1 2 3 1 2 Gel fraction (%) 90.5 89.091.0 92.4 90.8 Adhesion to substrate 3.02 2.50 3.72 3.65 3.60 (adhesionstrength, kg/5 mm) Processability 0.32 0.50 0.29 0.43 0.38 (currentvalue, mA) Corrosion resistance A A A A A Acid resistance A A A A AAlkali resistance A A A A A Bisphenol A Not detected 40 2,000concentration (ppm)

The aqueous coating composition of the invention is prepared using aspecific carboxyl-containing polyester resin, a specificlow-molecular-weight epoxy compound and a carboxyl-containing acrylicresin as starting materials. As a result, the composition contains nofree bisphenols and thus is free of the problem of environmentalhormones. Moreover, the composition has excellent applicationworkability, and is also excellent in film performance characteristicssuch as adhesion to substrates, processability, corrosion resistance,acid resistance, alkali resistance, flavor protecting properties, etc.

Therefore, the composition is suitable for coating the inner surfaces ofcans such as food cans.

What is claimed is:
 1. An aqueous coating composition comprising acarboxyl-containing, acrylic-modified polyester resin (B) neutralizedand dispersed in an aqueous medium, the polyester resin (B) beingobtainable by reacting a carboxyl-containing acrylic resin (c) and anepoxy-modified polyester resin (A) prepared by esterification of acarboxyl-containing polyester resin (a) having a number averagemolecular weight of 1,000 to 30,000 and an acid value of 5 to 200 mgKOH/g with a 1,2-epoxy-containing low-molecular-weight epoxy compound(b) which is a bisphenol A epoxy resin or a bisphenol F epoxy resin, theepoxy compound (b) having an epoxy equivalent of 140 to 250 and a numberaverage molecular weight of 250 to
 900. 2. An aqueous coatingcomposition according to claim 1, wherein the carboxyl-containingpolyester resin (a) has a number average molecular weight of 2,000 to10,000 and an acid value of 20 to 150 mg KOH/g.
 3. An aqueous coatingcomposition according to claim 1, wherein the epoxy-modified polyesterresin (A) is a resin prepared by esterification of thecarboxyl-containing polyester resin (a) with the low-molecular-weightepoxy compound (b) in such a ratio that the molar ratio of the carboxylgroups of the resin (a) to the epoxy groups in the compound (b) is 1:2to 1:1.
 4. An aqueous coating composition according to claim 1, whereinthe epoxy-modified polyester resin (A) has an epoxy equivalent of 2,000to 10,000 and a number average molecular weight of 4,000 to 100,000. 5.An aqueous coating composition according to claim 1, wherein thecarboxyl-containing acrylic resin (c) has an acid value of 100 to 500 mgKOH/g.
 6. An aqueous coating composition according to claim 1, whereinthe polyester resin (B) is a resin prepared by reacting theepoxy-modified polyester resin (A) and the carboxyl-containing acrylicresin (c) in an (A)/(c) weight ratio of 60/40 to 90/10 based on solids.7. An aqueous coating composition according to claim 1, wherein thecarboxyl-containing, acrylic-modified polyester resin (B) has an acidvalue of 15 to 200mg KOH/g.
 8. An aqueous coating composition accordingto claim 1, which further comprises a crosslinking agent.